There is no way for searching through text boxes in Calc and Writer. This is possible in Impress or Draw.
Steps to Reproduce:
1. Open Writer
2. Insert a textbox with some text
3. CTRL+F: search for the content
No results found
The text is also searched or add addition option to the Find & Replace dialog -> Other Options (similar to searching for comments)
User Profile Reset: No
Build ID: a9b202a6b7000e7af34f2a639ca207122a3968bf
CPU threads: 4; OS: Windows 6.3; UI render: default;
TinderBox: Win-x86@42, Branch:master, Time: 2017-12-26_23:10:23
Locale: nl-NL (nl_NL); Calc: CL
User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0
Yes, at least if the text is selected, search in the selection should be possible. Although an option to search in text in all shapes would be better.