Maybe it's to do with if at least one stat is over a certain value?
I noticed that Midgar Zolom with HP at 28k is unsensable with Vit and Mag at 255. Likewise, the same Zolom with everything (but HP) at 1 except level at 99 is also unsensable. However, returning that Zolom to it's original level it can once again be sensed.
So I'd say that HP, at the very least, isn't the ONLY factor.
Hm, Carry Armor has 38k HP doesn't it, but IIRC it can be sensed...