Welcome aboard Alopezh
I'm going to presume your playing SH3 stock/vanilla.
If this is the case then your snorkel will be detectable (especially by aircraft) because of the latter year availability and the obvious availability of centimetric radar by to the Allies.
If I were you I'd consider trying GWX3.0 because that comes ready equipped with a Late War Sensors and Snorkel Antennas mod.
The above will equip you with the following:
FuMB-3 Bali (Type VII and IX) from Oct44. Detection range/radar warning approx 7.5km
FuMB-37 Leros (Type XXI) from Jan45. Detection range/radar warning approx 9.5km