TRACKING INVESTMENTS (OR ANY TRANSACTIONS) ON STEEM
The answer is yes, you can check to see if someone has invested in STEEM, and you can go back beyond a few days.
The wallet used to go back a couple months at least, but for some reason, that history has been truncated for a few months now. It's a pain, but here's a couple of ways to look for it.
If you think a transaction may have occurred within the last eight days, you should be able to find it by going to steemworld.org/@theusername (where you put in the username of the person you want to check), then go to the Account Operations section selecting a day, and then filtering to IN and TRANSFERS. All of that can be found along the left hand side inside the Account Operations section. If it happened then, it should show something like this:
This shows that STEEM was bought via blocktrades, but it could be from some other exchange that allows the purchasing of STEEM.
If you can't find it within the last eight days, then the next best thing in my opinion is to go look for it at steemdb.com. Here, you can go straight to the website, then in the upper right hand corner of the home page, put in the username you're searching for in the CREATE ACCOUNT search window. You're not really creating an account, by the way.
Once the page for that particular user is up, you can click on the Activity tab in the menu just below the top menu, and then select transfers. It will bring up anything that's come into the account within a 90-day period. It's a little slow, so you might have to wait a little bit to access it. When the new page comes up, you'll want to look through the history for something similar to this:
In looking at it, it says 90-days, but it actually may give you way more than that (since my list actually went back to January 1 of this year).
There is a third option, steemd.com, but it doesn't filter like the other two do, but some people like it. I would suggest using it only if you're wanting to search for other things that happened right around that same time period. With steemd.com, you'll need to search through it all, and you might want to use Alt + Find with the word Transfers to single out the data because the list can be hard to read depending on what you're looking for.
The answer is yes, You can go through Steemd.com/@username and fish through all their history. It will be quite time consuming but it is there.
The closest tool I know of that tracks and filters out Wallet Transactions in a pretty neat way is Steemyy.com to which you can access it's Steem Account Transfer Viewer here: https://steemyy.com/transfer-viewer/
Just use any wallet exchange like @blocktrades. As an example here is all your transaction history with blocktrades sorted by amount. I think you can use a comma separator if you want to look at many exchanges all at once.
If you want to see how much was powered-up in a corresponding time-frame, use jeffbernst Steem Power Rankings: https://jeffbernst.github.io/steem-power-rankings/
Here is yours for example from Nov 15 2018 to Nov 24 2018. Although this tracks power-ups and not really investment! But powering up is also a somewhat investment so... XD