Hi kevhog49
Yes, you are right to be concerned. Checking the area in advance sounds a good idea, but the evening before sounds too early – hogs could easily move back into the area during the night. So I would suggest checking as soon before the mowing event as you can make it. That way, also, they would not need to be in boxes for too long – as you suggest a whole day might be too long for a lactating mother hog.
There is some information from BHPS
https://www.britishhedgehogs.org.uk/wp-content/uploads/2019/03/Hedgehog-Street-HEMP-guide.pdf Page 13 re. machinery.
I am fairly sure there is more information and if I find anything else will add it to another post.
I think the “ high-cut, low-cut method allows nest checks in-between, and increasing the blade height of mowers will minimise risks. Ensure all machine users are trained to be hedgehog aware.” Refers to cutting the whole area at a height of about a foot which makes it easier to see any hogs whilst at the same time, hopefully, the blades just missing them – then checking before the final cut (and putting any hogs in boxes until the mowing is finished and then releasing them nearby), but I suppose it depends how practical that is in a larger area.
Checking before as well would be good.
You could give BHPS a ring to see if they have any more ideas.
Good luck.