So another user (not sure if here or on reddit) made the remark that a connectiq field kept running in the background even after finishing the activity.
I just tested it by creating a connectiq field vibrating every 10 seconds and indeed, even after finishing and discarding/saving the activity (and being back on the watchface) the watch kept on vibrating every 10 seconds. When starting a new activity without this field this stopped. I can share the field for those interested, but there's not too much to it.
So I am guessing this is also causing the drain with the external sensor related connectiq fields, they keep on running in the background (trying to connect to the sensors) and thus draining battery till a reboot or starting a new activity without the affected connectiq fields.