ES keeps returning every document

I recently inherited an ES instance and ensured I read an entire book on ES cover-to-cover before posting this, however I'm afraid I'm unable to get even simple examples to work.

I have an index on our staging environment which exhibits behavior where every document is returned no matter what - I have a similar index on our QA environment which works like I would expect it to. For example I am running the following query against http://staging:9200/people_alias/_search?explain:

{ "query" : 
   { "filtered" : 
       { "query" : { "match_all" : {} },
          "filter" : { "term" : { "_id" : "34414405382" } } } } }

What I noticed on this staging environment is the score of every document is 1 and it is returning EVERY document in my index no matter what value I specify ...using ?explain I see the following:

_explanation: {
value: 1
description: ConstantScore(*:*), product of:
details: [
{
value: 1, description: boost
}, { value: 1, description: queryNorm } ] }

On my QA environment, which correctly returns only one record I observe for ?explain:

_explanation: {
value: 1
description: ConstantScore(cache(_uid:person#34414405382)), product of:
details: [ {
value: 1,
description: boost
}, {
value: 1,
description: queryNorm
}
]
}

The mappings are almost identical on both indices - the only difference is I removed the manual field-level boost values on some fields as I read field-level boosting is not recommended in favor of query-time boosting, however this should not affect the behavior of filtering on the document ID (right?)

Is there any clue I can glean from the differences in the explain output or should I post the index mappings? Are there any server-level settings I should consider checking? It doesn't matter what query I use on Staging, I can use match queries and exact match lookups on other fields and Staging just keeps returning every result with Score 1.0

I feel like I'm doing something very glaringly and obviously wrong on my Staging environment. Could someone please explain the presence of ConstantScore, boost and queryNorm? I thought from looking at examples in other literature I would see things like term frequency etc.

EDIT: I am issuing the query from Elastic Search Head plugin

In your HEAD plugin, you need to use POST in order to send the query in the payload, otherwise the _search endpoint is hit without any constraints.

In your browser, if you open the developer tools and look at the networking tab, you'll see that nothing is sent in the payload when using GET.

It's a common mistake people often do. Some HTTP clients (like curl) do send a payload using GET, but others (like /head/) don't. Sense will warn you if you use GET instead of POST when sending a payload and will automatically force POST instead of GET.

So to sum it up, it's best to always use POST whenever you wish to send some payload to your servers, so you don't have to care about the behavior of the HTTP client you're using.