The Tosefta says that anything involving monetary loss falls under returning a lost item. That also emerges from Rava’s statement about returning land, where too we are not dealing with an object that was lost but with saving property, and seemingly from his derivation one can learn precisely the point made by the Tosefta.
Rabbeinu Yonah in Sha’arei Teshuvah 3:70 says that this is part of the prohibition of ignoring it, which is puzzling, since on the face of it, based on the derivation, it is part of the positive commandment of returning it. And that also seems to be the view of Rabbeinu Bachya.
Maharam of Rothenburg says there is an obligation to remind a person that he forgot about someone who owes him money (Tashbetz Katan 495), and that this involves both a prohibition and a positive commandment. Likewise, the Rosh says there is an obligation to tell a person that he has a legal right of which he was unaware. And I think this was ruled in the Rema.
From here it follows that if there is benefit, there is an obligation from the perspective of returning a lost item and not only from the perspective of “love your fellow as yourself.” And one can discuss a case where the benefit is not monetary. If it is physical, then seemingly there is also returning a lost item with respect to his person, as stated in Sanhedrin.

The Talmud in Bava Metzia 26b says:
“Rava said: If one saw a sela coin that fell, and took it before despair with the intention of stealing it, he violates all of them: for ‘you shall not steal,’ for ‘you shall surely return them,’ and for ‘you may not ignore it.'”
“Love your fellow as yourself” and “do not stand idly by your neighbor’s blood” are not mentioned here, which implies that one who refrains from returning a lost item does not violate them. Perhaps that is because “do not stand idly by” speaks only about the laws of saving the person himself and not his property, as also seems to follow from Sanhedrin 73a, which Yishai mentioned. As for “love your fellow as yourself,” it may be that this is a commandment entrusted to the heart, like love of God, and cannot be fulfilled or nullified through actions. In other words, one can love someone and still hurt him on the practical plane, and conversely, one can fail to love someone and nevertheless help him in practice.

As for a prohibition on returning harmful information, does it follow from this that there is also a prohibition on returning a pack of cigarettes to its owner?

To Yishai,
If anything involving monetary loss falls under returning a lost item, why did Rava need to include returning land from the verse? It is specifically from there that one can infer that anything the Talmud did not explicitly include from the verse does not fall under returning a lost item, and therefore returning information would not fall under returning a lost item. As for the rulings of the halakhic authorities that you mentioned, did they bring proofs for this from the Talmud?

Oren,
What needs to be included are things that are not physical objects. I assume Rava would have said in exactly the same way, had he only been asked, “‘Any lost item of your brother’ — this includes lost information.”
As mentioned, the Talmud also brings the above Tosefta in connection with Rava’s statement, although it brings only the part about flooding water and does not continue to the end, “this is the general rule.”
The Talmud says that from the Tosefta, which speaks about building a fence against flooding water, and whose concluding clause is the general rule about anything involving monetary loss, there is no proof for Rava, because one could understand it as referring to sheaves standing in the field. According to this, the concluding clause too could be understood as referring only to movable property, and the novelty would be that there is an obligation of returning a lost item even when the object is not yet lost.
Still, once we already have Rava’s statement, it seems to me preferable to interpret the Tosefta straightforwardly and not force a strained interpretation about standing sheaves; then the concluding clause, “this is the general rule,” is not limited to movable property, and it indeed states Rava’s point, and then Rava’s own statement should also be understood more broadly.

In Sanhedrin 73a it seems that the obligation to return a lost item is grounded in “do not stand idly by your neighbor’s blood.” And the halakhic authorities wrote that where there is no obligation to return, there is no prohibition of “do not stand idly by.” See, for example, note 9 here: http://www.daat.ac.il/daat/kitveyet/assia/halperin.htm
And there is much more on this.

As for a pack of cigarettes, that requires discussion. Fattening food is also harmful, but it falls within what is permitted and is left to the person’s own decision. Therefore there is an obligation to return it. Regarding cigarettes, you have to decide whether in your view this is included in an actual halakhic prohibition or not. I am doubtful that it is truly prohibited, in addition to which the question depends on how much he smokes per day.