What is the level of danger you think this poses ? - that will depend what else is on that RCD, and perhaps on the power level and installation method of the PV inverter.

Are other things wrong?  - Are the  isolators and so forth suitable, is the overload protection appropriate etc.

Are there also batteries as part of the installation ?

Mike

Are you thinking along the lines of DC currents present and masking fault currents, preventing the RCD from operating?  

Are you thinking along the lines of DC currents present and masking fault currents, preventing the RCD from operating?  

If the inverter is capable of DC feedback