Non diabetic hypoglycaemia

You may be seeing "compression lows" where lying on the sensor whilst asleep can lead to incorrect readings.

If the lows haven't been confirmed with a finger prick test this may be a possible reason why you see what you do.

rivamira said:

I thought it might be those but it’s happened while I’m awake in the early morning and definitely not lying in the sensor. Thank you

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My sensors always read lower than a fingerprick. If you don't have symptoms and didn't double check with a fingerprick, I'd assume you didn't have a hypo.
It's also good to remember that it's normal for non diabetics to see numbers in the 3's, the threshold at 4 is for a safety buffer in diabetics on insulin or medication that can cause dangerous hypos.