Unless the ambient temperature varies sharply, and dramatically,over a very shrt period of time, PIR is still your best bet. PIR is used in all sorts of places where there are massive changes in temperature, (warehouses, houses, outdoors), without false triggering.
As was suggested prior, since you have mentioned that its a small box Capacative sensing will work, but I dont think that sensing something at a molecular level (cells) like you have asked about exists..its sems like a bit of overkill.
If it did, it may be so sensitive that its more prone to false triggering than PIR.