RPGMemes

As far as the detect magic thing goes, I think it depends a bit on the individual setting. I'm usually not playing in official WOTC settings, but per the PHB, a single cast of a low level spell can be 10-50 gp. A mercenary is 2 gp a day. Let's say for whatever reason that a spell caster was willing to take a midrange single spell cast payment for an entire day of work since it's a prestigious position, you're still looking at 30gp a day. That's equivalent to 15 mercenaries. That's just for a single spell caster who has 10 minutes of a spell that has a range of 30 ft, and can be blocked by the thickness of most castle walls. I don't know pricing difference for different wizard levels, but let's just say that you have level 1 wizards cast it as a ritual and have 2 wizards so they are switching off sentry/casting every 10 minutes. They work in shifts, so to be covered 24/7 you're looking at 3 8-hr shifts for two wizards per shift. So now you're employing 6 wizards, and you're still only getting 30 ft at a time. You could station them only in your treasury or whatever other important room, or you could have them walk the grounds on patrol. Unless you're switching to multiple sets patrolling you still won't be able to cover the full grounds all at once. At a certain point, security is just security theatre. Locks are a deterrent more than anything, so unless the person under attack is very very wealthy or is actively making high powered enemies, they probably don't actually need that level of security. I would guess that a place with less than 20 guards would not opt for that level of security. Anti-magic fields are not unheard of, and the above is certainly a thing that can be done, but I don't think most jails or bandit camps will have that level of security. Probably not even local mayors or city leaders.

If I was running a security heavy coronation or something, maybe the above would be relevant and the wizards would have something conferring invisibility, so the path they take would be unknown and random every time. Maybe they party heard through some rumors of an opening for a wizard position at the castle, though the monarch doesn't have a court wizard so they'd have a chance to know it's for security or something secret at least.

It might also be that it's unfun to make a heist where the shapeshifter is useless for stealth because every inch is covered by detect magic, or even if not every inch it adds unnecessary overhead of "where's the wizard casting now?" because someone has their familiar watching them anyway. No one is stopping you though, and I could definitely see it being fun to do for a big event.

As far as the guard thing goes, it probably depends on what is being guarded. A bandit camp in the woods will probably be accustomed to lots of animals. Maybe a castle grounds with a garden too. I think unless the location is known to house the Crown Jewels or something, the guards are probably just bored and aren't on high alert for those kind of things, especially if the PC is passing on stealth/nature checks to act natural. If a guard saw a butterfly in the middle of the castle in a room with no windows, that's cause for suspicion in a way that just seeing a butterfly outside would not be. Maybe a monarch would have overly zealous guards though.

Most pricing info from roll20: https://roll20.net/compendium/dnd5e/Expenses