Those sniffers are very senitive and the tip can be ruined/tainted real easy. Never stick it into the hoses or touch anything with them.
If freon get's on it most times they need to be replaced, the sniffer tips.

AC sucks to work on without a recovery machine. I have been busy with AC repairs here lately. I am working on AC as I type this.
Most of my trucks lose about a 1/2 pound per year.

I just pull it all out, pull a vacuum and recharge with 2.2 pounds and reset the dash light. Good for another year until I find it 1/2 pound low next spring.

Never any leakage visable around any connections or hoses.