FLIR ONE Pro LT
Find cold spots in your house
It's rare for any house to be completely without a draught.
See it on Amazon UK
See it on Amazon USA
See it on Amazon Germany/EU
This plugs into your phone and turns it into an infrared camera, capable of seeing those cold areas you'd not easily find otherwise. The link is for the one I have for the iPhone, but if you want it for Android, just select the appropriate model on Amazon.
These are affiliate links so I'll get a little money if you buy using them. Thank you!