Can a proximity sensor read through fabric?
You could try a Doppler radar sensor such as the ~10GHz motion sensor modules which are available on the surplus market and from China. 



As unlicensed and likely unapproved intentional emitters, they are probably of dubious legality to use, if that bothers you. They're probably safe to use (aside from possible interference with other electronics) but caution is called for- some people get sensitive if they think their DNA might get scrambled or their 'nads nuked.