You're most likely burning it. Could be leaking from head gasket, cracked head or possibly past a valve guide. If leaking past an intake guide it may or may not show in the spark plugs, it depends on how bad they're leaking. If leaking past an exhaust guide there would be no evidence in the combustion chamber as it would only be blown out your exhaust. It could be so slight you wouldn't notice steam from the pipes, it would only puff for a second at a time.