A mission note or tag is helpful, but many don't bother with it. Too, there are a lot of existing TB's that were out long before people started adding mission tags. Don't sweat it, if you see you cannot help its' travels, simply don't harm its' travel either -- just drop it before taking too far off course.
That said.... you will find that some people don't seem to give a hoot anyway. Oftentimes TB's are dragged in the opposite direction. So... yes, there is an etiquette, but not all bother with it.
You can take all the TBs that you can tote. The idea is to move them, not to leave them languish.
Side-Note: The tracking # is important (as you discovered). It is also important to hold that tracking # until your logging is complete and finished properly. It is oh-so often that people mislog, only later to find they cannot correct it 'cuz they don't have the number (duh!).
Thanks for the info. I will keep note of the tracking # for the one we have got, just in case we get the log wrong when we set it free.