You're misunderstanding: the companies doing the investing, if they're publicly traded companies HAVE to report these sorts of things in their finance reports. This is why when these companies buy out other companies they make announcements about amounts, and if they've sold off anything, this, that and the third.

When I worked for KeyW Corporation I had shares with the company, and every time they did anything they made announcements, like if they acquired someone. I had left the company and during my leave Jacobs Engineering bought out KeyW, sold off some of their own business endeavours and paid off all of KeyW's debt and shifted their focus towards government contracting and less so in oil infrastrcuture (if I remember correctly, Jacobs is a big company). This was all publicly announced.

The startup doesn't need to disclose anything, but if a public company invested in them, they will eventually have to do a quarterly report and disclose it.

Even a non public company has to post some business numbers, depending on the jurisdiction.

And in these business numbers is usually some type of (rough) balance sheet,and on balance sheets you will see your holdings in other companies. (because accounting standards require it)

Just for underlining your "not posting is always an option" ;)