Tosafos explains that there are 3 types of places. 1. If an object is found in a place where it isn't at all protected it has regular rules of an aveida - with a siman it must be returned, without a siman we assume the owner was miya'esh so the finder can keep it. 2. If an object was found in a place where it is well protected then it should not be touched, and even if the finder accidentally takes it he can return it to the place that he found it (unless there is a chance that the owner came back and didn't find it there, in which case he must track down the owner and return it). Regardless of whether it has a siman or not, it shouldn't be touched. 3. If an object was found in a place where it is partially protected, but there is a possibility that it will be damaged - with a siman it should be returned, but without a siman he should leave it there.
In the third category where the place is semi-protected and the object doesn't have a siman, so that it should have been left there and not touched, if the finder accidentally picked it up he must hold on to it and not return it to it's original location. This is the concept of ספק הניח לא יחזיר, meaning that if you aren't sure if it was intentionally placed there or lost, you shouldn't touch it, but if you took it you can't "return it to it's original place". Why? Because you become a watchman over it, which makes you responsible to return it to the rightful owner.
Why in the case where the area was only semi-protected does the finder who picked it up assume status of a shomer that prevents him from placing it back down, but when the area is fully protected he doesn't become a shomer and can put it back down? Tosafos holds that the finder becomes a shomer who is responsible to do whatever would be in the best interest of the owner in both situations. If the area is well protected it is in the best interest of the owner to have it placed back down, but if the area is not well protected then it is in the best interest of the owner that he hold on to it.
Rashi here and later on daf 37b understands the words ספק הינוח לא יחזיר to mean that the finder cannot return it to the person who claims to be an owner and fails to provide a proper siman. The reason is the same - since he is a shomer on the item he has a responsibility to hold on to it until he finds the person who can prove himself to be the owner, and not return it to the wrong person. Tosafos asks on rashi that it is pashut that it shouldn't be returned to the person who claims to own it without a siman, just as with any aveida - why does it have to say it? Perhaps rashi holds that since it is an object that doesn't have a siman, there is almost no way for the owner to get it back (unless he brings witnesses that it is his). Therefore it is in the best interest of the owner that the finder would return it to whoever claims to be the owner even though he doesn't provide a siman, because that would be the only way that the rightful owner could possibly get it back. The statement of ספק הינוח לא יחזיר teaches that since the finder becomes a shomer, he is not allowed to return it unless he is sure to have found the rightful owner.