keep somebody's seat, etc. warm
keep (one's) (something) warm
To occupy a position for one or take over their responsibility while they are absent. Usually used in the form "keep (one's) seat warm," and often followed by "for (one)." I'm don't usually work in reception—I'm just keeping Mary's seat warm while she's on sick leave. Bob asked me to keep his place warm while he's on vacation, so, if you have any questions, you can direct them to me.