If you are a dull muppet who keeps leaving their keys at work (WHO would do a thing like that...) or, worse still, have an other half who puts down the wrong keys, leaving the house with both sets of car keys and no house keys... (cue Laurel & Hardy theme tune) this could be the answer!
This product does exactly what we want from it, and it seems of pretty sturdy construction.
Although probably just another measure of my dopeyness, I would say that when researching your potential purchase be aware that there is a version with the rubber back (this one) and one with wall-mountable screw-fixings.
Also, don't be misled by the picture: it might give the impression of being quite compact (due perhaps to presence of the credit card) but it's actually about 1/2 A5 in size. However, don't let this put you off. We have shackled ours out of public view but where there is clearly no right of way or implied permission to enter uninvited, and where neighbours would quickly spot a trespasser, so we're very pleased with it.
Now if we could only remember the code........