On my 2002 Sienna, several years ago, I had intermittent 0442 and 0446. Sometimes it would go months without a failure. Then, it would fail a lot.
I did not take it to a mechanic, because the odds of it failing while the $85/hour clock is running are very slim.
I replaced the cap, more than once. I also left the code reader on it, so I could see when it failed. And, I Googled to the point of nausea.
Finally, on the old Sienna Club a man said he had the exact thing, and owned two Sienna's. He swapped parts until he found the fix. it was the canister assembly. He said it looks like it had some low pressure, self-actuating valves, and he theorized one might have been sticky.
I took my Sienna to the dealer and told them I was taking responsibility for the troubleshooting, since I did it for 31 years. Problem has not come back on the passing years.
So, you need to find out if it is truly intermittent. Note when it failed, and if it comes back again, note that as well, until you get a handle on it. That is, learn how intermittent it is, or if it was a one time thing, as others have said is possible.