Time Machine, among its other significant weaknesses, is ill suited to offsite backup – especially when it is used with Time Capsule.
If you attach an external drive to Time Capsule there is an “archive feature” that will shut down TC access and safely transfer the data to an external drive. Problem is, neither the drive nor the backup are encrypted.
Carrying around non-encrypted backups is not a good idea.
There’s a similar problem with a standard Time Machine external drive. If you swap them, you run into the same encryption problem.
The answer for an external drive is to use an encrypted disk image and mount that for TM backups. That doesn’t, however, work with standard Time Capsule archive behavior. I suspect one might be able to disconnect all clients and use the finder to copy the disk images to an encrypted disk image, but I’ve not tested that.