I have a 2002 Jeep Grand Cherokee with a little over 80,000 miles on it. I have had it for abou2 months and have had to refill my coolant reservoir back up twice from the add line. I had it pressure tested and block tested and those came back fine but the mechanic could not find a leak? Could it just be that small that they are missing it? Or is more likely that something is going on internally? Thanks for the help.

Try a new cap first, it could be weak. But sometimes those jeeps with coolant reservoirs just lose some in the reservoir from evaporation IF block leak test was done correctly