Hi William,
This morning (Sunday) I heard a young girl shout "Glass! Glass!" from the pool.
I went out to have a look & there was a family with many young children around the pool, trying to avoid standing on a glass "mug" that had smashed into many sharp pieces.
It was far enough away from the pool that hopefully no pieces actually went into the pool, but there is really no way of knowing.
I took these pics before I swept it up with a brush & pan.
I didn't hear or see the incident, did you happen to recognise any of the culprits? There is one of those big security camera's pointing directly at the area where the smashed glass was found, so their identity could be easily found, especially with your "timestamp" William of ~6.30pm yesterday evening.