Time machine eventually complains about a missing time capsule

In my environment I have the following scenario.

- Several apple computers are running ML in my network. All of these mashines are configured to use time mashine to do backups to the same set of time capsules.

- I do use 3 (three) time capsules for backup. One is always stationary, while the 2 others are exchanged in a fixed interval (a couple of weeks). One

time capsule is always at an external place. The plan is to have a a maximum data loss of a couple of weeks.

Everything is almost running fine. Every hour time mashine is doing backup up to a different disk. The missing time capsule is skipped. But over the sudden

sometimes a time mashine dialog appears, telling me that it could not backup for so many days because the backup disk is missing. The problem is,

there will be no further backup until this dialog is ackknowledged.

I have no idea what situation will trigger this event. I do fear that my unattended server will stop backing up and I do not realize it. Or is this triggered by

some application program which interacts with time mashine?

Any comments are welcome.