Lockdown mode takes very expensive equipment to bypass
When you reboot, the user data encryption key is flushed from memory, and your unlock code is necessary to decrypt the KEK (key encryption key) held by the TPM / SE chip (which also applies rate limiting). All common attacks on unpowered devices are bruteforce with rate limit bypass.
In lockdown mode the KEK stays in memory so app state and user data is there, but the CPU is in a state which prevents access to it until unlocked with your code. There’s a few more attacks possible, but most are completely blocked by the fact that USB data connectivity is also disabled in this state. You practically have to open up the device to pull data out.
Tldr no ordinary thief will even try.
The bigger risk is having it stolen while unlocked. Make sure you have revocation options ready for keys held on the device
They too have an understanding of object permanence for things outside their field of vision. They want to understand what’s near them and what’s touching them.
If they realize the thing touching them is unknown and it can’t make sense of what it might be then it might be something dangerous (and this instinct will still be there in a safe space in your home).