It's still truancy.
Thinking of the children we've had on Safety Plans where the instant they aren't in class, somebody is notified for an urgent search that mustn't stop until they have been found or you're pretty sure they're not on site, you've then got to cover;
Medical.
Main building, three floors, three sets of stairs, library, four sets of toilets, four entrances/exits/Learning Support. SENDCO's office. Library. Respite rooms.
Seven labs, three prep areas, staff toilets.
Four Deputy's Rooms, one head of year's office, one counsellor's office.
Dining Hall, toilets, kitchen and delivery area.
Back of PE block, changing rooms. Three gyms and associated storage rooms and cupboards, toilets.
Food Tech, back of food tech. Food tech storage room.
Main Hall, back of hall, behind stage, top of stage.
Art rooms. Prep Room. Store Rooms.
Behind Art Rooms and Prep Room.
Music practice rooms (12). Drama. Music classrooms. Performance space, behind stage, under stage, check all cupboard doors.
Corridor leading to more toilets. The toilets.
DT area. Toilets.
Sixth form, toilets, common room, dining room, classrooms.
Disabled toilets.
English huts. Behind each English hut.
Languages building and toilets.
Computer Rooms.
Behind plant room.
Field 1, Long Jump field, back of field. Rugby pitch.
Tennis courts and astroturf.
Approximately 490 separate trees, bushes and other vegetation.
Front car park, middle car park, top car park. Behind vehicles, between vehicles.
Back of bins.
Back of kitchen bins.
Behind caretaker's shed, storage and office.
Go to IT, find where the technician is at this moment (probably under a desk fixing a computer right now), get them to return to their office and start looking through CCTV of each entrance to see whether they've nipped out when a delivery has arrived. Check the fence line to see whether they've gone over it at some point around the three mile perimeter.
All by a small number of people who should also be dealing with other things because they don't just sit around all day waiting for one kid to not show up in lesson. And add to that the anxiety of checking each area, wondering if this is the day you're going to find a child in need of urgent lifesaving first aid - whilst the suitably motivated kid is often moving to a new location ahead of you because they've heard footsteps or kept lookout and know you're searching for them.
Sending an automatic within 5 minutes' alert just results in parents screaming down the phone about how you're overreacting/frightened them when the kid usually walks into class at the same time as the screaming down the phone line commences - or you're trying to go and look for them but you're stuck on the phone being screamed at, so aren't able to look for them until the parent hangs up.