On shabbat, in an eruv: Is it acceptable to pick up an ownerless object (a free book for instance) and move it to a slightly less exposed place in the public domain, with the hopes that it will be there to acquire after shabbat? The object would still remain ownerless for this time, and would not be concealed, but would have less of a chance of being picked up by another person.
It is permitted to move the item, where this does not involve a great effort, so that others will not pick it up.
Although it is generally forbidden to perform acts on Shabbos for the purpose of Chol — this is considered hachanah, preparation from Shabbos to the weekday — it is permitted to perform acts that will save a person from loss, even where the item in question will not be of any use on Shabbos.
This principle emerges from a number of sources in Chazal, such as the Gemara that permits a person to place a cover on a muktzeh item in order to save it from being ruined, or another Gemara permitting a non-muktzeh item to be moved from the sun to the shade, even though it has no use on Shabbos. See, at length, Machazeh Eliyahu (Falk, no. 46), who expounds on this prinicple.
In this case, although the acquisition of an item from hefker is a question of “gain” rather than saving from loss, it would nonetheless appear that moving the item will be defined, for the purpose of muktzeh, as saving from loss.
The picking up of the item by another passer-by will is seen by people as a “loss,” and although technically it can be classified as the prevention of gain, it appears that the general outlook of people will be the significant classification for purposes of muktzeh.
Therefore, it will be permitted to move the item, to prevent the “loss” of others picking it up. It is not permitted, however, where this involves a major effort, because of the principle of “tircha” on Shabbos.