Can Bitcoin full nodes pinpoint the true source of a transaction which they've relayed? Assume I'm running a full node and that I've relayed some transactions. As a full node owner, can I pinpoint the true source of the transactions I've relayed?
In the bitcoin data directory there is a file called debug.log.
This file as noted in the bitcoin documentation:


  May contain IP addresses and transaction ID's.


This does not indicate that a transaction originated from a specific IP address but it does indicate that your node received a transaction from that IP address.  If someone were to have a wide enough view of the network and controlled enough nodes then it could be inferred that the creator of a specific transaction was using a specific IP at the time.  This is not always considered conclusive evidence at this time.