Mother pen keeps disappearing for weeks or days, is this normal?

My workplace has a lake onsite with a pair of mating swans that have been with us about 5 years.

This year they have 3 surviving cygnets who are now around 4 months old. The pen disappeared and the cob stepped in and took care of them. We feared the worst but then she came back after 2 weeks.

Since, she has continued to disappeae and come back. Is this normal swan behaviour as this seems to be the first year she's behaved like this and is there any reason she would she be doing this?