Deja Vu. I left mine at T3 security on the way to Madrid some years ago.

When coming back to LHR, i went to Lost Property and was able to reclaim it, but to do so I had to turn on the computer to prove it was mine.

So I would make sure before you call Lost Property that you are armed with your mother's passwords as you will likely be asked to do what I did before they will forward it on to your mother.

All the best.