For air leaks I've always gone with soapy water to find the spot. But not on a fish tank... lol
If it's not the sump water being too low, then I'm guessing the air leak is occuring between the sump and the pump. If there was a leak happening between the display and the pump then water would likely be leaking out, not air getting in.
I'm surprised the silicone on the connections didn't fix the problem. Not that it would be a for sure difference, but did you apply the silicone while the pump was off and give it some time?
If any of your pieces are threaded, you could try adding more teflon tape. I had salte creep leaks through threads and extra teflon cured it for me.
Good luck!