In the Name of Allah the Beneficent the Most Merciful
If a person loses something in the masjid by way of a child picking up that which does not belong to him/her and it is something that the one who lost it needs in order to get back to his home which is not extremely close to the masjid like the keys to his/her automobile how can one find the item if he cannot ask if anyone has seen them?
Zayd, Allaah reward you with good. I think you are alluding to the fact that it is impermissible to make 'lost&found' annoucements in the masjid? What he can do is; speak to one of the admin. outside of the masjid to enquire about it and a poster can be posted outside of the masjid (such as in the hallway or foyer of the Islaamic centre/book shop etc. as long as it isn't in the masjid itself).
On a similar note, what should the Imaam (or admin.) of a masjid do when they find a lost item? Should they keep it for one year etc. like a lost item in other areas? We asked the Egyptian Scholar (who is a long-term resident of Saudi Arabia) Shaykh Muhammad Ibn 'Abdul-Wahhaab al-Bannaa about this some years ago, he mentioned that if we are unable to locate the owner in a reasonable amount of time, then let it belong to the masjid thereon.