Not 100% sure love but I know you can test positive on an OPK after trigger Tried and tested by moi!
Actually, now that you mention it my clinic did say not to do a HPT before my BT with them (Yeah, right!!!) as I might get a false + from the Ovidrel.
We all metabolise these things at different rates though love. Mine was out of my system at 6dp5dt (and could have beeen out earlier for all I know) and I've heard of others still detecting it 14 days later....