I don't know. To my ear, "it is in use" implies continuous use. From what you say, the use is intermittent.
Ironically, the continuous tense in this case works better to suggest intermittent use.
That is, we can say "It's being used from time to time."
We'd be less likely to say, "It's in use from time to time," although it's certainly possible.
So I think "it is being used" is more appropriate in this case.
When you say "it's in use," it may be taken to mean "it's being used continuously at the moment, but eventually it will not be in use." (So if you wait a while, you'll be able to use it.)