1. On the S-Meter, even with a low signal report, I get the "OV" light up, is this normal, and should I worry about it?
It indicates ADC overload, if you look on the spectrum display you may find some broadcast signals are above the -30dB line. An attenuator, or better still notch filters will help with this.
2. Occasionally the web page (with my SDR) stops with "audio underrun", I have to refresh the page, and it's back to normal. Is this happening because I am using something else on my PC? (running through Chrome). Checking the log, does show this error message.
It happens occasionally some browsers are more prone to it than others.
3. I notice the tags at the top of the frequency, is it possible to easily to add my own tags, or are they imported from a source file?
Shift left click on the banner above the waterfall.
4. I need to put the GPS antenna outside, as it does not show any acquired signals.
The supplied antenna is not the best. It does work, but it's much better if it is placed outside.
They are French pseudo CB'ers / 'Freebanders' / Echo Charlie operators.
There are unlicensed operators from loads of different countries using that segment of the spectrum. 6660.00KHz LSB is typically used as a calling channel.
Other frequency segments also get used by 'Pirate operators' they very often stick to fairly narrow frequency limits which may partially be because they use simple resonant antennas and don't want to have to use a tuner. Some of these 'Pirate' bands have been running for many years.
Some examples include:-
'Zender' stations using AM and broadcasting traditional music from 1600KHz to 1800KHz.
Russian CB'ers using ex military equipment mainly between 2900KHz to 3200KHz AM /USB / LSB and calling on 2920.00KHz
Filipino fishermen speaking in Tagalog 16540/16545/16550KHz USB
Could be Indonesian village radio 10940KHz +/- 100KHz USB