coolant leak from level sensor

[fo3]

I've been dealing with a small coolant leak for a while, like 200ml every month. It's taken a long time to track it down and I'm almost sure it's the level sensor (but could be the overflow tank I guess).
There's a c channel bar running under the rad and after degreasing everything and taking it for a 20km drive there was spots of blue coolant there, with the wiring going to the sensor feeling wet.

Reason for posting a thread is how common is it? If not common then I'm possibly barking up the wrong tree and the leak could be somewhere else (like expansion tank or lower rad hose there). Will just changing the oring inside it help or do these sensors commonly crack and leak in the plastic anyway?

I searched this topic and all mentions of the coolant level sensor were about it not reporting right, and replacement ones being useless and failing a lot. Also a lot of talk about expansion tanks were about how replacement ones don't seal right and fail a lot. My current expansion tank and sensor seem fine so I don't want to replace OEM with shoddy aftermarkets if I don't have to. I just want to do the minimal to fix this tiny leak and I had a feeling it's the internal oring, but if that isn't a common fault then I may be wrong on that...

On that note is the coolant level sensor part of the rad or expansion tank? IE I mentioned before about the fact there's a c channel on the chassis below it, is that removable or does moving the rad help or removing the expansion tank help is order to get room enough to pull the level sensor out from the bottom - what's the procedure for a m50 in a e34? There's nothing much in the bentley or factory manual about doing this small job of just replacing o ring or the level sensor in-situ.