Carbon monoxide alarms sounding in my rental

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I'm really struggling to know what to about this and would appreciate some advice. In a nutshell, I have CO alarms sounding in one of my rentals, despite a clean bill of health from my gas engineer, and don't know what to do next.

In more detail - I have two CO alarms in the property (both long-life ones with 3+ years left till expiry); one upstairs near the gas boiler and one downstairs near the gas cooker. The downstairs alarm sounded (once) in March, for no apparent reason; my gas engineers investigated it, found nothing amiss and renewed the gas safety certificate. Fast forward until this week, and this time the upstairs alarm has sounded, again just once.
So the fact that both alarms have now sounded means that a faulty alarm is highly implausible. Yet, the gas system on the property has been pronounced OK too. Clearly I have a responsibility to my tenants to investigate though.

I'm now thinking the most likely scenario is CO getting in from outside somehow. Traffic fumes is not a likely culprit; this is a quiet suburban street. Is there anything else I'm missing? It's a terraced house, so I have to wonder whether it's coming from next door (either side)? I've asked my tenants to make enquiries there but haven't heard back. But given that I have zero rights over next door, where does this leave me? It's hardly appropriate for me to commission a gas check in those properties, or install CO alarms. Just wondering where to go from here.