Here are some pointers and photos for how to find the main Heathrow lost property office. This is where all lost property will end up after a few days. The office in T5 shown above will hold on to items for only a short time since space there is very constrained.

The office is inside the Heathrow Express station for T3, which is a short distance from the main T3 building (and shorter distance still from T3 arrivals). It is actually after the barriers for the services, so if you are trying to visit the office from landside you will need a free inter-terminal ticket from the machine to get in and out. You may want to mention something to the Heathrow Express agents at the ticket barrier just in case there are any problems getting out. If you are travelling on the Heathrow Express anyway, then you can stay within the ticket area and you don't need to take any special actions, other than that you do not go through the ticket barriers at T3. The office itself is one level below the tunnel / barriers but one level above the platform. The Tube station is some distance away, the two are linked via tunnel.

This is the view from the landside tunnel that links T3 departures, T3 arrivals to Heathrow Central bus station and Tube station. And at the main entrance to the Heathrow Express you can see the yellow sign for Lost Property (or Lost and Found if you prefer).



You then go through the barriers and down one level to the office. This would be the view if arriving at T3 by HEX - you don't want to get out of those barriers.



And this is the office door itself, one level down from the barriers, at the time of writing it is open 07:00 hrs to 19:00 hrs 7 days a week. There's no buzzer to press to enter, just enter the doors shown in the photo, and there is a desk with a bell on it. There are a few seats if they did to hunt around for your item but if you have already made contact with them beforehand they will probably have the item readily to hand for you.