The trouble is that plenty of parents will say 'Oh, she's constantly eating' when they haven't seen the easily portable food items folded up and stuffed in a pocket to be disposed of later or the quick visit to the bathroom with the tap running to hide the sounds, especially when there's just a small snack (which could well be given to somebody else) at lunchtime and then a tiny meal in the evening because it's cooked by somebody who eats tiny meals. They also tend not to notice subtle changes and weight loss over time because they see their child every day, rather than somebody who knows them but sees them every couple of weeks or so in completely different situations and up against other children of the same height.
Kids will also talk amongst themselves and sometimes you'll find out only when one child discloses 'competitive' or group disordered eating that there are actually 3-4 others who are acting the same way but have been more successful in concealing it. Even extremely gentle investigating can see those in tears and trying to get parents to back them up, not because they're fine but because they are in the grip of an ED and want their parent to say 'no, of course she eats tons, she's always eating' so they can continue.
It's also very possible to notice trends in behaviour, such as wanting to go to the toilet during lesson time straight after lunch, being seen sitting with nothing whilst friends eat or even being heard throwing up on more than one occasion when walking past on a corridor patrol.
Of course, defending in innocence is better than the laughter from a parent at the thought that her 'fat' child could possibly have an ED - she was admitted to hospital shortly afterwards. And I took no pleasure in witnessing the shape a leg can go when it breaks from malnutrition when that parent could no longer say 'Oh, no, we're just naturally small' for another kid. Quite pleased with the child who we called an ambulance for, knowing that the parent would say she just needed a snack because she didn't eat that much that turned out to have undiagnosed T1 Diabetes, though, as a) they didn't die and b) she looked much healthier and happier once on medication.
They're concerned about her health and wellbeing, not doing it deliberately to make her cry and you to feel like they're indirectly picking on you.