It is possible to enter the (landside) Arrivals Lounge without going through any security, albeit with an appropriate used boarding pass.

I hope I'm not the only one to see the risk of leaving unaccompanied and potentially insecure baggage there. (Not saying there was a risk in this particular case.)

As I mentioned before, there is a stored baggage facility landside in T5 which should surely be the appropriate place for left baggage. Here's the link: http://www.heathrowairport.com/heath...s/left-baggage