There are cases such as for HLT_mu6_xe30_mht_L1XE10 where the chain can be seeded in events with no L1 muons, this is because only MET is included in the L1 item.
As muon are aligned before MET, the muon Step runs first and is responsible for rejecting the chain in the ComboHypo at the end of the 1st muon Step.
But as there is nothing to process in L1-muon-less events, the InputMaker flags a warning.
As this is understood - the warning is demoted to debug.