Transaction id from a multisig address
It seems that you figured it out yourself so I just want to add that we are going to add multisig support for mainnet as well in a few days and you will be able to use that as well.

BUILDING means that the transaction has been included in a block and it's confidence is "building". There is also a field confirmations which you can use as well.

The multisig functionality is pretty new to the site and that's why we haven't documented it yet. It will be done asap.