In my recently followed IPSC RO Course I was referred to the "IPSC Range Master Guide" and specifically: "Range Equipment Failure", which depends on RO decision under Rule 4.3.1.6, mandatory under Rule 4.6.2, Rule 9.1.5.2 and Rule 9.1.6.2
For all our IPSC L2 matches the RM always marks the exact location of the start positions, barricades, target stands, popper locations, mounted targets with clearly marked chalk, ducttape, etc. The Scorer and RO verify the positions after every squad.