Dealing with no data
From your question it seems that you are easily able to identify the periods assets were down for maintenance and simply wonder how to best code that in the data?

If that is the case I would simply add a new column of information marking assets as either 1 - active or 0 - down for each period. If need be you could even code it into active/down/maintenance, etc.

This would be additional valuable data for your prediction model (maybe assets are more or less likely to go down after a maintenance period). Additional it would allow you to simply mark the sensor data as missing NA because you will be able to differentiate between a broken sensor and an asset on maintenance from your other data.

As @Tasty213 suggested any good classification algorithm might have been able to do this as well but it seems you are simply able to add this data yourself.