There seems to be quite a bit of confusion. First you should tell us if the squelch is for a VHF/UHF set, or some HF equipment. They are very different!
A VHF or UHF squelch works by detecting the disappearance (or at least lessening) of noise. That means you can't turn the knob clockwise for increasing sensitivity. The 'most sensitive' point is the point where the squelch is just on (below) the limit between letting the noise pass or not. If you connect an antenna, when the squelch is just open (i.e. lets the noise pass), it can happen that it then closes because of the extra noise coming in. It's just doing its job.
An HF squelch on the other hand, generally works by measuring the signal strength. Its most sensitive point is also between 0 and max, but increasing the noise with the connection of an antenna, generally tends to 'open' the squelch.
In both cases, the 'most sensitive' point is the limit between opening and closing the noise.
EDIT: As you update the question to specify it's a VHF radio, a 'de-activation' of the squelch (i.e. the passing of the noise), may indicate that some specific frequencies are absent from the antenna noise, which makes the squelch think that there is a valid signal on the input. This could be something like electric noise (possibly from fluorescent tubes), or some other source. Could be even some broadband signal such as an oscillation.