it's also possible the other agencies only shared findings rather than specific records.

For example if the law says "plate reader records cannot be shared" and the CHP just confirms the presence of the records , and does not share the records, no violation occurred.

You did a good job reading the article from bottom to top. The headline and lead are usually misleading.

One question is whether OPD violated state law by leaving their data open for anyone to search.

When they signed up to these systems, did they know that federal agencies could search their data without OPD needing to do anything?

But from my quote, it sounds like federal agencies couldn't actually search the data themselves. Rather the Feds would do something like make a request to the Sacramento PD (random example), and then Sacramento would then search the OPD database, and then, presumably, give that data to the feds.