I dunno... everytime I use Spell Trap, I do not seem to gain back any spells even though I get hit by spells. I have not done extensive experiments on this, but this is my general perception.

I think i know why.
Like any other spell protection (excluding Spell Immunties), Spell Trap only absorbs spell that are targeted, and do not affect an area.
For example, if your image will target yourself with an ADHW spell, you will NOT gain a 8th level spell back, but the image will target you with a Flame Arrow, for example, you WILL get a 3rd level spell back.

BTW Dundee - your new avatar is AMAZING!