An experienced traveler taught me this trick. I altered all my travel T-shirts on her advice.

Gone are the days when, when traveling, men hid valuables in their underpants, as in the movie “Love and Doves,” and women in their bras. Although this guarantees the safety of valuables.
Now there are special handbags with a strap that can be worn under clothes, wallets on the wrist, things with zippered pockets. The main thing here is to keep everything valuable with you. The thieves have not died out and their skills have improved.
You also need to be vigilant and careful on the road. You can't trust anyone.
Don't drink alcohol with anyone, even alone.
Don't trust anyone with your things.
Don't play cards.
Do not leave drinks unattended; they may add sleeping pills.
Do not fasten your pockets with a pin so that it is visible.
I had an incident at a market in Moscow. I fastened the side pocket on the skirt with the money with a pin, right at the top, I think I’ll cover it with my hand just to be safe. Whatever it was, they climbed in, unfastened it, didn’t notice anything, but didn’t have time to pull it out. People will shout: “Thieves!”
For trips you need to have things with pockets to feel your valuables, for such cases I have a T-shirt with pockets. I put on a loose shirt on top so that nothing is visible. If I go to bed on the train, I sleep lightly. I feel my “jewels” (passport and money) and put my phone under the pillow.
How and where to sew pockets. It’s best from the side, so that when you press your hand unnoticeably, you can feel everything.
My T-shirt has a pocket at the bottom for my phone, since that's what I use most often. I made it without any fasteners. Above there is a double pocket.
To do this, I folded a strip of fabric into 2 unequal parts. I sewed a zipper on top of the passport pocket.
Second pocket for money and bank card, Velcro closure.
You can hide it.
Nothing is visible.
Everything is safely hidden in a hiding place, you can enjoy your trip, look at the beautiful views outside the window and not worry about your things.