I am not aware of any warrant canary existing at the moment.
Monero (unlike ZCash and Dash) doesn't have any central authority - an organisation, that could receive the subpoena secret order or equivalent. Each node in network is equal to the rest of nodes and nodes are running opensource code that is developed in the open on GitHub. Multiple people monitor the development and each code addition must go through pull request which is reviewed by some core dev and this whole process is visible to everybody. The government would have to target individual developer in their country, presumably core dev doing code review of some malicious pull request. It is quite unlikely but not impossible that this will ever happen.
It is good idea that xmr.to, mymonero etc should create a warrant canary for their services. And probably core devs too, just in case. However unlikely the situation is.