Why am I receiving empty envelopes? With small pieces of parchment in them?

A week ago received a letter to our house with the same name as my dad's name on the top left corner and an address which isn't ours, same street but a different door number.
Someone had crossed out the number and written 'try…' -our door number.
Opened it, and it had a small blank piece of white paper in it. STRANGE.
Thought nothing of it threw it away.

But today received same envelope this time with our address on, again dad's name, same hand writing, AGAIN with a small blank piece of white paper:/ (papers are same size as an iPhone, little bit smaller and both times were slightly diff size/shape)

What on earth is this?! Is someone doing this deliberately you think? So strange!

Strange yes

if addressed to your father - what does he make of it

assuming he doesn't live with you, why would they be trying other than his address

odd

almost seems like the sender believes the intended recipient will know what to make of it, a cryptic message, a threat?

of course. Maybe not for your father at all

someone else with the same name?

could the paper be concealing something or be infused with a drug?