On some of the units, the bottom layer of flooring had vinyl tile with traces of asbestos and in many of them the mastic had asbestos. 

Never use "premixed mortar," which is really just adhesive mastic with sand in it.

Asbestos wrap usually appears a little thicker - this could just be paper and mastic (of course, the mastic could contain asbestos).  

Most buy and holders I have met are doing deals in the Shirley/Mastic area.

I had some black mastic on my concrete from some old floor tiles.

They used the premixed mastic but the 300lb guy (who left his cell phone on the window sill) couldn't believe the mastic had not set in 3 hrs so he could walk on it.

That house could have it in stucco, window putty, drywall joint compound, ceiling texture, sheet vinyl flooring, vinyl floor tile, flooring and other mastics, HVAC ducts, and other things - most major surfaces.

The only way a home would have asbestos is that someone found and used some old asbestos tile mastic, other old building material or some such remote chance scenario.

The sub used a 1/4-in. square notch to sparingly spread budget mortar for 18-inch square tiles and dropped them in, because r'aal professionals don't need no stinkin' backbuttering and swirling spreading is good enough for mastic, so it should be good enough for mortar.

I've had luck in shirley/mastic area.