R134a into the passenger compartment?

[Andrew Sitzer]

Hey all,
 I was wondering if anyone has any advice on what would cause R134a to
leak through the vents into the passenger compartment.  My mechanic's 134a
sniffer was off the charts (5 of 5 bars) when he put it to my vents..
 He insisted that it isnt anything to worry about, but I disagree -- this
stuff is flammable, poisonous, and bad all around.
 The system itself functions fine (nice cold air).. and we didnt pick up
any leakage in the engine compartment; though the system was
pretty much empty after about 2 yrs of use.
 I am thinking the evaporator could cause this -- anything else?
Also, am I just over-reacting?

Thanks in advance!
Andrew