Bad smell like kerosene/paint thinner in upstairs of property - unable to identify

I am the owner of a semi-detached house.

Last week an extremely strong smell started (a kerocene/oil/paint thinner smell), it is concentrated in the master bedroom to the rear of the house and strongest in the walk-in wardrobe in that room. The smell is also present (and stronger) in our neighbour's house, concentrated in their master bedroom (which backs onto our master bedroom).

We left the property for a night due to the smell, upon return the next day it had subsided somewhat, but was still present. 4 days later our carbon monoxide alarm went off (it was located our master bedroom).

We have oil fired central heating. The oil tank and boiler are located in the back garden. We have had our central heating boiler checked by our plumber who could not find any problem.
We had a pressure test completed on our chimney - which passed with no issues.

Our neighbours have had their central heating checked, have had a builder visit the property and will have their chimney cleaned tomorrow.

We are completely stumped.

Any ideas?

Are there any wall vents near the flue of the oil boiler?

The oil boiler is about 16 feet from the back door, there are wall vents but there is no smell in the kitchen (the closest room to the boiler).