This is too alarmist, @Lawrence.
Pardons don’t have to be announced or publicized by noon tomorrow, but like all official acts, they have to be documented by noon, and Congress can subpoena all pardons on Thursday to get a final record. https://twitter.com/lawrence/status/1351386288658677760

As an example of the Framers’ concern about executive secrecy, the Secret Treaty of Dover between Charles II and Louis XIV was suspected and scandalously discovered years later in 1770s and condemned by the Framers during the impeachment debate on July 20, 1787.

Congress can issue subpoena for all White House pardon records and to private citizen Trump himself. If they have no official record or provide no official record of such a pardon, then it doesn’t officially exist.

I was googling for the Presidential Records Act text as @abosc33 replied below.
Congress can’t legislate away the pardon power, but I’d suggest an official document, to be considered official as a factual/legal matter, has to comport with the PRA:
https://www.law.cornell.edu/uscode/text/44/2201

Answer to where the subpoena should be sent is also an answer about what makes presidential acts official by law, and how they have to be documented to be official documents:
@abosc33 gets it: https://twitter.com/abosc33/status/1351655113887199235

5/ Irony:
IIRC, Congress passed Presidential Records Act b/c of Nixon's secrecy & secret Oval Office recordings.
One reason Nixon installed automatic recording was to keep audio of presidency as his own personal property and claim its donation as a tax write-off!
cc: @leoncrawl